Nowadays, vehicle passenger protection systems, also known as vehicle safety systems, usually comprise airbags arranged in various frontal and side locations of the passenger seating positions. Airbags consist of a flexible envelope which is designed to inflate rapidly through pyrotechnic means during a crash of the vehicle, in order to prevent passengers from being severely injured.
A typical airbag system includes proximity and impact sensors (which detect when an accident situation is going to occur), squibs, and a control module which may include a microprocessor or a microcontroller. During an accident situation, the control module provides energy for deploying the squibs, which in turn ignite pyrotechnic material to inflate the airbags.
For safety reasons, it cannot be assumed that, in an accident situation, the power source of the vehicle, i.e., the battery, remains active during the crash. Also, the wiring harness of the vehicle may be damaged so that the airbag system may be disconnected from this power source. Therefore, airbag systems generally include an energy storage device such as a reservoir capacitor which is intended to store sufficient energy so as to supply a current allowing to heat each squib instead of the battery whenever necessary. Thus, the airbag system may rely on the reservoir capacitor to operate during crash situations without any supply from the car.
It is desirable to diagnose the value of this capacitor in order to validate that enough energy is available for firing and ensuring deployment of all airbags. In extreme cases, the capacitor can get disconnected (for instance, if the soldering quality is faulty) and this diagnostic allows detecting it.
In prior art solutions disclosed, e.g., in WO 2006/046218, the diagnosis may be done by causing the reservoir capacitor to be discharged by a current I of fixed value, resulting in a voltage decreasing with time. By measuring the time interval Δt for a fixed voltage drop ΔV, or by measuring the voltage drop ΔV for a fixed time interval Δt, the capacitance value C of the reservoir capacitor may be determined as:C=I·Δt/ΔV 
However, such solutions necessitate that the current I be accurately known and maintained stable during the measurement. This may prove difficult when the capacitor is discharged under conditions which vary during the capacitor measurement with the operation of the load, thus yielding in inaccuracy of the diagnostic. In addition, the variation of the load current during the capacitor measurement is creating additional errors.
In order to circumvent these difficulties, one may think of causing the reservoir capacitor to be charged through an external resistor by a current of a known value, the RC value being calculated from a measurement of the time in between two voltage set points. Knowing the external resistor value, the capacitor can be computed. In order for the measurement to be substantially independent from temperature, the external resistor must be of rather high value. Such additional external component increases the cost of the system.